Stock-Based Compensation - Stock Options Outstanding and Exercisable (Detail) CAD / shares in Units, CAD in Millions	12 Months Ended
	Dec. 31, 2017 CAD CAD / shares shares
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options outstanding, Number of options (in shares) | shares	1,479,275
Options outstanding, Weighted average years to expiration	5 years 8 months 12 days
Options outstanding, Weighted average exercise price (in dollars per share)	CAD 150.64
Options outstanding, Aggregate intrinsic value | CAD	CAD 117
Options exercisable, Number of options (in shares) | shares	796,348
Options exercisable, Weighted average exercise price (in dollars per share)	CAD 119.32
Options outstanding, Aggregate intrinsic value | CAD	CAD 87
Stock options in-the-money (in shares) | shares	1,479,275
Stock options in-the-money weighted average exercise price (in dollars per share)	CAD 150.64
Stock options weighted average contractual Term	4 years 10 months 24 days
Range of exercise prices from $36.29 - $117.48 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of option exercise prices, minimum	CAD 36.29
Range of option exercise prices, maximum	CAD 117.48
Options outstanding, Number of options (in shares) | shares	416,270
Options outstanding, Weighted average years to expiration	3 years 8 months 12 days
Options outstanding, Weighted average exercise price (in dollars per share)	CAD 84.50
Options outstanding, Aggregate intrinsic value | CAD	CAD 60
Options exercisable, Number of options (in shares) | shares	416,270
Options exercisable, Weighted average exercise price (in dollars per share)	CAD 84.50
Options outstanding, Aggregate intrinsic value | CAD	CAD 60
Range of exercise prices from $117.49 - $165.95 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of option exercise prices, minimum	CAD 117.49
Range of option exercise prices, maximum	CAD 165.95
Options outstanding, Number of options (in shares) | shares	359,997
Options outstanding, Weighted average years to expiration	6 years 9 months 18 days
Options outstanding, Weighted average exercise price (in dollars per share)	CAD 141.38
Options outstanding, Aggregate intrinsic value | CAD	CAD 32
Options exercisable, Number of options (in shares) | shares	200,362
Options exercisable, Weighted average exercise price (in dollars per share)	CAD 128.43
Options outstanding, Aggregate intrinsic value | CAD	CAD 20
Range of exercise prices from $165.96 - $190.42 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of option exercise prices, minimum	CAD 165.96
Range of option exercise prices, maximum	CAD 190.42
Options outstanding, Number of options (in shares) | shares	415,914
Options outstanding, Weighted average years to expiration	6 years 1 month 6 days
Options outstanding, Weighted average exercise price (in dollars per share)	CAD 182.48
Options outstanding, Aggregate intrinsic value | CAD	CAD 20
Options exercisable, Number of options (in shares) | shares	106,458
Options exercisable, Weighted average exercise price (in dollars per share)	CAD 169.28
Options outstanding, Aggregate intrinsic value | CAD	CAD 6
Range of exercise prices from $190.43 - $222.88 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of option exercise prices, minimum	CAD 190.43
Range of option exercise prices, maximum	CAD 222.88
Options outstanding, Number of options (in shares) | shares	287,094
Options outstanding, Weighted average years to expiration	6 years 8 months 12 days
Options outstanding, Weighted average exercise price (in dollars per share)	CAD 212.05
Options outstanding, Aggregate intrinsic value | CAD	CAD 5
Options exercisable, Number of options (in shares) | shares	73,258
Options exercisable, Weighted average exercise price (in dollars per share)	CAD 219.60
Options outstanding, Aggregate intrinsic value | CAD	CAD 1